The present invention relates to a magneto-optical recording medium capable of recording information and reproducing the same by utilizing magneto-optical effect.
Conventionally there is known a magneto-optical recording medium capable of recording information and reproducing the same with application of a light beam such as a laser beam.
In such a magneto-optical recording medium, the recording of information is performed by writing information with a light beam in the form of reversed magnetic domains in a perpendicularly magnetized magnetic thin layer, utilizing the optical thermal effect of the applied light beam, and the recorded information is read from the magnetic thin layer by utilizing the magneto-optical effect thereof.
When the recorded information is read from such magneto-optical recording medium, or the writing of information therein or the deletion of information therefrom is performed with application of a laser beam thereto, a guide track for accurately guiding the position of the application of the laser beam is necessary. Such a guide track is particularly important when information is recorded with high density, since the guide track can accurately locate the position of any small recording pattern so that reliable tracking servo can be attained.
The conventional guide tracks are grooves formed in the recording medium, having, for example, a pitch of about 1.5 .mu.m to about 3 .mu.m, a depth of about 1/8 n x.lambda., and a width of about 0.5 .mu.m to about 0.8 .mu.m. The guiding of the laser beam is performed by utilizing the diffraction of light.
In the recording layers of the conventional magneto-optical recording media, transition metal--heavy rare earth element amorphous alloys such as TbFe, TbFeCo, GdFe and GdFeCo are in general use, and the reproduction output (C/N) thereof has recently exceeded 50 dB, partly because of the recent improvement of a pick-up and other devices. However, these recording layers are extremely susceptible to oxidation. In order to improve the durability of the recording layers, it has been proposed to add some additives such as Al and Ti to the recording layers or to provide a protective layer on the recording layer. These methods, however, are not yet sufficient for attaining the desired improvement of the recording layer.
Under such circumstances, magnetic oxide films, made of, for example, barium ferrite, cobalt spinal ferrite or iron garnet, having semipermanent durability and large magneto-optical effect, have been proposed. Such magnetic oxide films can be prepared by the sputtering method, the thermal decomposition method, and the vacuum deposition method, provided that the substrate temperature must be generally set higher than in the case of the ordinary fabrication of magneto-optical recording media. Therefore, generally employed plastic substrates cannot be employed when such magnetic oxide films are formed, so that a heat resistant substrate such as glass substrate has to be employed.
When a plastic substrate is employed, tracks can be formed without difficulty, for instance, by using a conventional method such as the injection method. However, it is difficult to form a track directly in a glass substrate.
In the case of a glass substrate, the grooves are generally formed by the etching method. However, when the etching surface of the substrate is uneven or not smooth, the crystal orientation of a magnetic oxide film formed on the substrate is adversely affected by such uneven etching surface. The result is that the C/N becomes poor.
In the magneto-optical recording medium using a magnetic oxide film, the Faraday effect of the light transmitted through the magnetic oxide film is utilized. Therefore, it is preferable to obtain a large Faraday rotation angle by making the magnetic oxide film as thick as, for instance, 1,000 .ANG. to 10,000 .ANG..
However, if such a thick magnetic oxide film is formed on a substrate having grooves, the groove portions from which tracking signals are actually generated are deformed by the formation of the magnetic oxide film thereon. As a result, the track servo and C/N deteriorate.
Japanese Laid-Open Patent Application No. 59-98332 discloses a magneto-optical recording medium comprising a flat substrate, a magnetic oxide layer formed thereon, a transparent intermediate layer formed on the magnetic oxide layer, a synthetic resin layer having step portions on the transparent intermediate layer, and a reflection layer on the synthetic resin layer. In this reference, it is disclosed that by appropriately setting the thickness of the step portion of the synthetic resin layer, and the thickness of the other layers, and by forming reflection-preventing areas and non-reflection-preventing areas, tracking servo signals are obtained. In this structure, the magnetic oxide layer is formed on the flat substrate so that the above mentioned shortcoming of the conventional magneto-optical recording medium is eliminated.
However, in the above-mentioned magneto-optical recording medium, the thickness of each layer has to be strictly made uniform within a predetermined range. However, it is extremely difficult to form each layer with a predetermined thickness accurately over a large area as in a disk-shaped recording medium. Furthermore, the above-mentioned synthetic resin layer is formed with a photo-resist. In such a case, there is the risk the grooves formed in the synthetic resin layer may be deformed with time by the heat generated with application of laser beams thereto in the course of writing information and deleting the same.
Japanese Laid-Open patent application No. 57-183647 discloses a magneto-optical memory element comprising a substrate, a magnetic thin layer formed thereon having an axis of easy magnetization perpendicular to the surface of the magnetic thin layer thereof, and band-shaped reflection layers on the magnetic thin layer. In this reference, it is disclosed that as the magnetic layer, layers of transition metal--heavy rare earth amorphous element alloys such as TbFe and a layer of MnBi can be employed. However, these magnetic layers are easily corroded and the above-mentioned band-shaped reflection layers cannot be prepared by the Wett method utilizing photolithography in spite of its high productivity.
Japanese Laid-Open patent application No. 57-40768 discloses a magneto-optical recording medium including a transparent protective layer, and a metal overcoated layer having a melting point which is lower than that of any of the magneto-optical recording medium and the transparent protective layer. In this magneto-optical recording medium, guide tracks are formed by vaporizing the metal overcoat layer in the shape of a band with application of laser beams thereto. However, as long as this method is employed for forming guide tracks, the productivity of the magneto-optical recording medium is poor. Even if it is tried to etch the metal overcoat layer by photolithography, it is difficult to employ the Wett method having high productivity without deteriorating the magnetic recording medium since it contains an amorphous alloy film made of, for instance, TbFe. The same thing can be said even when a protective layer is provided.